This is a program for designing and yet the way you can get and select additional fonts is very very unpleasant.
For every font that you want to test, you always have to click onto that drop down button and then select one. Actually it should be possible to use Up/Down keys and there should maybe also be some Up/Down buttons to click onto.
After you selected the fonts, they get already added to the project, which doesn’t have to be. People maybe want to select some fonts and have them only in the program-assets for a while until their decision is done. And after that you should be able to select the one(s) you want (right now you have to uncheck the uncomfortable toggle-buttons one by one instead of being able to select many or all at the same time for disabling them.
I also think after adding the fonts, on the right side in the appearance-settings you should be able to use Up-/Down-Keys to change the Font among the available ones.
Also the automatically added (default) Fonts should be visible at any time!
But the idea to use Google-fonts is brilliant never the less! Keep it up, thank you!
P.S.: Where you can see the fonts with a default Text, it must be possible to either automatically set that text to the Element you were or are just working on and or manually set the text and it’s font size and color (much better would be to actually see the text at the Element in the Design you are currently working on)